A very heavy sea this morning. It is drizzling with rain but there is no wind. A storm out at sea has brought a wreck of petrels scattered up and down the beach, left there as the tide has receded. In the space of the two or so miles I walk every morning, there must be thirty or forty birds. I guess they are petrels but I have to say sea birds are not my strong point and I have no idea which species. Most of them are blue grey above and white underneath and in size not much bigger than a blackbird or thrush but with startling cobalt blue legs.
The next day with friends from the local branch of the Ornithological Society, we picked up more at Ohiwa spit. My friends readily identified the birds. There was the odd Kerguelen and a few tiny grey-backed storm petrels but the majority were blue petrels, Halobaena caerula, the white tipped tail being the confirming mark of identification. Members of the Ornithological Society pick up dead seabirds as a matter of course and record them. In this way a check is kept on the sea birds.
There are apparently 97 species of smaller petrels and of these 49 have been recorded in New Zealand seas. The strategic position of our islands strung across the prevailing westerly winds accounts for the many varieties of petrels found in this sector of the Pacific Ocean. The multitude of small islands scattered around New Zealand provide suitable nesting habitat for some 32 species of petrel which breed from the Kermadecs in the North to Campbell Island some 2400 miles to the south. Other species such as the blue petrel visit New Zealand waters on a regular basis.
The distribution of seabirds is related to the physical and chemical properties of seawater and to the ocean currents. The tropical, subtropical, sub Antarctic and Antarctic water masses are characterised by specific planktonic organisms and the birds that feed on them. By travelling clockwise around the Pacific many petrels can make efficient use of the local food supply as it comes to fruition. However, when these local food supplies fail for one reason or another, then they may end up wrecked on beaches.
On still nights the southern oceans can twinkle with the tiny lights of phosphorescent zooplankton rafted in millions upon the water. Since many of these animals disappear during the day, most petrels feed at night. During the summer when the sun never sets, enormous blooms of the crustacean, Euphausia superba, attract vast concourses of sea birds to the cold polar seas. Some of the smaller petrels can devour a quarter of their body weight in the space of a few minutes so that they become so grossly distended they cannot fly.
To pick up food from the surface of the water, they face into the wind and hold their trembling wings outstretched to keep themselves aloft while trailing their legs in the water to prevent them from being blown backwards. In this position they hover, sometimes making pattering runs as they peck at the food in front of them. It is this “walking on the water” which gives them their name of “little Peters” after St Peter; hence petrels.
Petrels breed once a year in large colonies on remote offshore or oceanic islands. Most of the main mountain ranges in New Zealand used to support extensive petrel colonies before the advent of man a thousand years ago and the mammalian predators they brought with them.
The Blue Petrels are circumpolar, breeding on many sub Antarctic islands but not in the New Zealand region. They are sedentary during the summer breeding periods but then range widely through the southern oceans between the pack ice and about 40 degrees south. Diet is mainly crustaceans, small fish and squid, taken from the sea surface by dipping in flight or while the bird is swimming. They occasionally follow ships or feeding whales.
The ornithologist W.R.B. Oliver describes the Blue Petrel on the Kerguelen Islands, “The hillsides apparently quite deserted during the day, became at night perfectly alive with these birds. The burrows are excavated beneath the mounds of an umbelliferous plant, Azorella selago, which abounds on the hillsides. The holes turn sharply to the right or left, parallel with the hillside, thence downward often doubling once or twice on itself and communicating with other entrances. At the bottom is an enlarged cavity lined with root fibres, twigs, ferns or leaves, and quite dry. Here the single egg is to be found always covered with dry powered earth or leaves.”
The petrel family, which includes shearwaters and albatrosses, has tube shaped nostrils at the base of the beak which allows them to process seawater. Unlike most other birds, except the turkey vulture, they have an excellent sense of smell. Just as scent guides them to their own nest burrows at night during the breeding season, so it helps them to find food out in the open ocean. When fish or shrimps feed on plankton the tiny plants release a chemical substance and it has recently been discovered that even the tiniest whiff of it will attract petrels.
Petrels have been companions to countless seafarers and for many island dwelling communities such as the Maori an important food supply. However, in most parts of the world mutton birding, the collection of the fat chicks, is more of a tradition than a necessity today.
The petrels are vulnerable in two respects. They need security of nesting places free from predators such as rats. Still more important is the continuing productivity of the oceanic food chain. Increasing contamination and pollution of this resource and overexploitation, if not checked, will spell their doom.
Posted: Sunday, October 31st, 2010